I’ve been searching on the Internet to try to find out what people mean by “supreme truth,” who say that all paths lead to it. Most of the search results that came up were about a terrorist organization with that name. Most the ones that I found saying that all paths lead to it, did not say what they mean by it. One of them equated “supreme truth” with God. Another compared it to a mountain top. Some others describe it as some kind of experience, or state of being.
I’m wondering now if people are thinking of it as something to know, or something that can happen to them, that will make them as happy as they could ever want to be, forever.
As I see it this is the crux of the problem you have keyed in on. I'll try to to explain this as I see it.
It has to do with a dualistic view of reality, versus as nondual view. In a dualistic reality, there is you, and then there is the world out there, outside of you. "Truth" in dualistic systems of thought are viewed as truths outside of yourself, independent of you and your subjectivities.
So when someone speaks of "Ultimate Truth", or Reality, it is still viewed as external to oneself. Ultimate Truth is viewed, as any other mundane truth, as a propositional statement that one tries to comprehend with the mind. So as statement such as, "You shall know the truth and the truth shall set you free," is seens as learning some propositional truth, some doctrine, some fact or another to "believe in" or comprehend with the mind of reason.
On the other hand, in nonduality, Ultimate Truth, or Reality, is not a propositional statement. It is the true nature of everything. It is not about beliefs. It is literally beyond beliefs, beyond rationality, beyond any mental comprehensions. We ourselves are not separate from Ultimate Reality, but are ourselves that very Truth. To "know" that, is not a mental belief, but a Realization of who and what we are, which is That which is everything. It is something which is apprehended by our being, as opposed to comprehended by the mind.
So when you ask someone who is exclusively seeing Reality through a dualistic lens, "God", like your car or your friend, is "outside of you". When you ask someone with nondual experience and realization about it, they will say that Truth is "just this", and point to everything that is, including themselves.
To the question then of all paths lead to that Ultimate Truth, that is true. The end of all dualistic paths, when reached to the end of dividing the world into this versus that statements, reaches a barrier where you can go no further until that barrier, or veil, is dissolved. Then you see Ultimate Truth, and it is everything that is. "Just this". Not a satisfying answer to the mind trying to figure itself out.